I used Corrin on Ivy. The dragon vein for fliers is an AOE healing zone and the Quality time passive is nice so it's been fun using her to heal up instances where theres a lot of chip damage on everyone. I'm sure some people would rather use her offensively, and maybe it's more optimal. But this was also a great and easy way for me to grind supports for her since I really wanted to read them. Oh, and I had canter on her, so that she could exploit Corrin's debuffing passive to chip away at a boss and move out.
I also really liked Byleth on a sage Pandreo. Mine had amazing magic and speed growths, but he was much more physically fragile than Ivy, so I preferred to have the extra Thrysus range on him so he could safely snipe things from afar.
This wasn't functionally amazing or anything but I ran Micaiah on Mage Knight Alear for a laugh and it was pretty funny throwing him out into the middle of nowhere with Nosferatu equipped and everything he didn't manage to dodge he could heal himself right back up.